Hydrostatic interbody

ABSTRACT

An intervertebral implant includes an upper surface configured for engagement with a first vertebral body, and a lower surface configured for engagement with a second vertebral body. A wall extends between the upper surface and the lower surface, and forms a chamber for containing osteogenic material. At least a portion of the wall is collapsible from a first position associated with a first volume of the chamber to a second position associated with a second volume of the chamber. The second volume is less than the first volume.

FIELD OF THE INVENTION

The present invention relates generally to implants for placement intobone recesses, and more specifically to interbodies for dynamicallytransmitting loads while promoting fusion between bones.

BACKGROUND OF THE INVENTION

In spinal fusion, two or more vertebrae are joined by additional bonematerial placed between the vertebrae. Once fusion is complete, the bonematerial immobilizes the vertebrae. Spinal fusion is used primarily totreat pain caused by abnormal motion of the vertebrae. Anterior lumbarinterbody fusion (ALIF) is a spinal fusion technique that can be usedfor treating degenerative discs from an anterior approach. The anteriorapproach allows access to the interbody space with minimal damage to theposterior musculature, while allowing full decompression of the diseaseddisc. During an ALIF procedure, an interbody device is inserted withinthe intervertebral body space. This interbody is generally composed ofPEEK or titanium with a central opening for bone graft material, whichis typically an autograft or allograft material. The objective ofinterbody fusion is to fuse the central graft material to the cranialand caudal endplates, creating a rigid boney union between motionsegments.

Known interbody designs have a propensity to stress-shield the graftmaterial. That is, the interbodies, or the fasteners used to anchor theinterbodies, absorb axial loads during settling of the implant. This hasthe effect of shielding the graft material from axial loads. Someinterbody designs are configured to expand in an axial direction afterbeing implanted to increase the height of the disc space to a desiredspacing. This also stress-shields the graft material, and actuallyremoves load from the graft material because the height of graft spaceexpands.

SUMMARY OF THE INVENTION

In a first exemplary embodiment of the invention, an intervertebralimplant includes an upper surface configured for engagement with a firstvertebral body, and a lower surface configured for engagement with asecond vertebral body. A wall extends between the upper surface and thelower surface. A chamber, which is enclosed within the wall, includes anupper end opening through the upper surface, and a lower end openingthrough the lower surface. The wall includes a collapsible sectionbetween the upper surface and the lower surface. The collapsible sectionis collapsible from a first position associated with a first volume ofthe chamber to a second position associated with a second volume of thechamber. The second volume is less than the first volume.

In a second exemplary embodiment of the invention, an intervertebralimplant includes an upper plate configured for engagement with a firstvertebral body, and a lower plate configured for engagement with asecond vertebral body. A chamber extends between the upper plate and thelower plate. The chamber contains an osteogenic material under ahydrostatic pressure in the chamber. The upper plate is axially movabletoward the lower plate to reduce the volume of the chamber and increasethe hydrostatic pressure on the osteogenic material in the chamber.

In a third exemplary embodiment of the invention, an intervertebralimplant includes a body formed of a shape memory material. The body isdeformable in response to temperature from a pre-insertion configurationto a post-insertion configuration. In addition, the body forms a chamberand contains an osteogenic material in the chamber. The chamber has afirst volume and exerts a first hydrostatic pressure on the osteogenicmaterial in the pre-insertion configuration. The chamber has a secondvolume and exerts a second hydrostatic pressure on the osteogenicmaterial in the post-insertion configuration. The second volume is lessthan the first volume, and the second hydrostatic pressure is greaterthan the first hydrostatic pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description will be more clearly understood in conjunctionwith the drawing figures, of which:

FIG. 1 is a perspective view of a first exemplary embodiment of aninterbody in accordance with the present invention;

FIG. 2 is an exploded perspective view of the interbody shown in FIG. 1;

FIG. 3 is a bottom view of a superior component of the interbody shownin FIG. 1;

FIG. 4 is a top view of an inferior component of the interbody shown inFIG. 1;

FIG. 5 is a perspective view of a second exemplary embodiment of aninterbody in accordance with the present invention;

FIG. 6 is a perspective view of a third exemplary embodiment of aninterbody in accordance with the present invention;

FIG. 7 is an exploded perspective view of the interbody shown in FIG. 6;

FIG. 8A is an exploded side view of the interbody of FIG. 6 with aportion cut away to expose an interior component in a first condition;

FIG. 8B is an assembled side view of the interbody of FIG. 6 with aportion cut away to expose an interior component in a second condition;

FIG. 9 is a perspective view of a fourth exemplary embodiment of aninterbody with biologic material in accordance with the presentinvention;

FIG. 10A is a schematic cross-sectional view of the interbody of FIG. 6,shown in a first condition;

FIG. 10B is a schematic cross-sectional view of the interbody of FIG. 6,shown in a second condition;

FIG. 11A is a schematic cross-sectional view of a fifth exemplaryembodiment of an interbody in accordance with the present invention,shown in a first condition; and

FIG. 11B is a schematic cross-sectional view of a fifth exemplaryembodiment of an interbody in accordance with the present invention,shown in a second condition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

Interbody implants in accordance with preferred embodiments of theinvention address a number of interests. One interest is to provide arigid structure that maintains proper spacing between vertebrae. Asecond interest is to minimize the pre-implantation height of theinterbody, so that the interbody can fit into compressed disc spaces. Athird interest is to provide an interbody that provides sufficient spacefor graft material and promotes fusion of that graft material.Applicants have observed that these three interests frequently competewith one another. Moreover, Applicants have observed that knowninterbodies fail to balance and satisfy all three interests. Many knowninterbodies appear to disregard the third interest, namely the interestof promoting fusion of graft material in the implant. This interest iscommonly sacrificed in favor of the one of the other competinginterests.

To promote fusion of the graft material within the interbody, theinterbody should allow some load to be maintained on the graft material.A consistent loading on the graft material is important during thefusion process to encourage bone growth in the bone tissue. Theimportance of maintaining load on graft material is rooted in Wolff'sLaw. Under Wolff's Law, healthy bone will adapt to loads it is placedunder, and will remodel itself to become stronger if the loadingincreases. Conversely, if the loading on a bone is decreased or removed,the bone will gradually become weaker. That is, there is no stimulus forcontinued remodeling of the bone to maintain bone mass. In the contextof spinal fusion, Wolff's Law holds that applying consistent loading tothe graft material promotes fusion.

To balance the competing interests described above, the embodiments ofthe present invention provide structures that dynamically transmit axialload to the graft material during interbody subsidence, while providinga rigid structure to maintain proper disc space height. Duringsubsidence, the bone graft material is confined within the chamber andis compressed under load. As a result, hydrostatic pressure develops inthe bone graft material, with pressure bearing on the material frommultiple directions, including the axial and radial directions.

The preferred interbodies in accordance with the invention include acentral chamber filled with osteogenic material. For purposes of thisdescription, “osteogenic material” includes but is not limited to anymaterial that promotes bone growth or healing, including autograft orallograft material, or synthetic graft material. The osteogenic materialis maintained under compression to form a solid fusion between theadjacent vertebral bodies.

In contrast to interbodies that are designed strictly to expand afterinsertion into the disk space, preferred interbodies in accordance ofthe invention include a contraction mechanism that allows theinterbodies to contract under load over time, reducing the volume of thechamber containing the osteogenic material. Decreasing the size of theinterbody over time promotes fusion of the osteogenic material byapplying a constant pressure on the material. The chamber radiallyencloses the osteogenic material, so that the osteogenic material has noroom to expand or migrate during an axial contraction of the interbody.This has the effect of applying a constant pressure both axially andradially around the osteogenic material. Constant application ofpressure, or a gradual increase in pressure as the case may be, promotesfusion of the osteogenic material under Wolff's Law. Because theembodiments of the invention maintain or increase hydrostatic pressureon the osteogenic material, fusion of the bone material is promoted.

Contraction mechanisms in accordance with the invention may take one ofseveral forms that allow the interbodies to collapse or shrink withrespect to one or more planes of reference. The contraction mechanismsare designed to contract in response to changes in loading on the spine,subsidence of the interbody into the end plates of the vertebrae,resorbing of the osteogenic material, or changes in temperature. As agraft material resorbs into the body, for example, the volume of thematerial may decrease and no longer be under hydrostatic pressure in thechamber. In such cases, the contraction mechanism allows the interbodyto collapse by a controlled amount to reduce the volume of the graftspace and maintain constant compression on the graft material. Thecontraction mechanism can be designed to maintain equilibrium betweenthe osteogenic material's resistance to compression, and the loadsbearing on the interbody.

Referring now to FIGS. 1 and 2, an implant 10 in accordance with a firstexemplary embodiment of the invention is shown. Implant 10 includes anupper plate 20 and a lower plate 30. Upper plate 20 has an upper surface22 forming an exterior surface on the implant, and a lower surface 24forming an interior surface of the implant. Similarly, lower plate 30has a lower surface 34 forming an exterior surface on the implant, andan upper surface 32 forming an interior surface of the implant. Upperand lower plates 20 and 30 both have ring-shaped bodies that surroundhollow interiors. When upper and lower plates 20 and 30 are joined orstacked relative to one another, the hollow interiors align to form acentral chamber 50 for containment of an osteogenic material 80. Thesuperior and inferior end plates adjacent implant 10 form the upper andlower walls of chamber 50.

As noted above, implants in accordance with the present inventioninclude a contraction mechanism that facilitates a controlled rate ofimplant collapse. Contraction may occur solely in the “axial” direction,represented by axis “A” in FIG. 2, the “radial” direction, which is anydirection perpendicular to axis “A”, or a contraction on both the axialand radial directions. Implant 10 includes a telescoping contractionmechanism 60 that permits upper plate 20 to collapse axially towardlower plate 30. A plug or shaft 62 extends from lower surface 24 ofupper plate 20. Lower plate 30 includes a socket 66 that receives theshaft 62 during contraction of implant 10. Shaft 62 is generallycylindrical and forms a bore 63. Bore 63 and socket 66 collectively formpart of chamber 50 for containing osteogenic material 80.

Shaft 62 is configured to slide telescopically into socket 66 duringcontraction of implant 10. As upper and lower plates 20 and 30 collapseinto one another, the volume in chamber 50 decreases. In thisarrangement, contraction mechanism 60 is operable to reduce the volumeof chamber 50 over time and maintain compression on osteogenic material80. In the preferred embodiment, implant 10 includes a mechanism forlimiting relative rotation between upper and lower plates 20 and 30.Referring to FIGS. 2 and 3, for example, shaft 62 includes a pair oflobes 64. Lobes 64 mate with a pair of diametrically opposed notches 68in socket 66, shown in FIGS. 2 and 4. Notches 68 telescopically receivelobes 64 as shaft 62 enters the socket 66. The sliding engagementbetween lobes 64 and notches 68 maintains radial alignment between upperand lower plates 20 and 30, and substantially prevents rotation of oneplate relative to the other to stabilize implant 10.

Interbodies in accordance with the invention preferably includesurfacing to promote engagement with end plates of vertebral bodies.Referring to FIG. 2, for example, upper plate 20 includes a plurality ofridges 23 and lower plate 30 includes a similar plurality of ridges 33.Upper and lower plates 20, 30 are anchored into adjacent vertebrae witha plurality of bone screws 90. It will be understood, however, that anumber of fastener types may be used to anchor the plates, including avariety of screw sizes and configurations.

Referring now to FIG. 5, an interbody 110 is shown in accordance with analternative embodiment of the invention. Interbody 110 includes aone-piece body 115 having an upper plate section 120 conjoined with alower plate section 130. Body 115 forms a central chamber 150 forcontaining an osteogenic material. Contraction mechanisms 160 areprovided in the walls of body 115 to allow upper plate 120 and lowerplate 130 to be collapsible in an axial direction relative to oneanother. Each contraction mechanism 160 includes a wall section with alarge aperture 164 and an elastic member 168 contained in the aperture.Apertures 164 form thinned sections in upper and lower plates 120 and130 that deflect in response to axial load. In this configuration, upperand lower plates 120, 130 are permitted to collapse axially relative toone another when subject to axial loads. Elastic members 168 providelimited resistance to contraction and absorb some of the axial load.Apertures 164 are separated from adjacent fastener holes or otherapertures by hinge portions 166 that allow the plates 120, 130 tocollapse.

Elastic members 168 provide a further benefit by absorbing some of thecompressive load and protecting against end plate failure. That is, eachelastic member 168 counteracts the compressive force and reduces thetotal net force on the osteogenic material and reaction force on the endplates. Elastic members 168 further allow interbody 110 to self-distractafter insertion into the disc space. Distraction may occur by mechanicalexpansion of the elastic members, or by thermal expansion in the casewhere the elastic members are formed of shape memory materials.

Referring now to FIGS. 6-8B, an interbody 210 is shown in accordancewith another alternative embodiment of the invention. Interbody 210includes an upper plate 220 telescopically received in a lower plate230. Upper and lower plates 220, 230 form generally rectangular ringbodies with open center areas that collectively form a chamber 250 forosteogenic material. Interbody 210 further includes contractionmechanisms 260 in the upper and lower plates 220, 230. Lower plate 230has three hollow sidewalls 232 a, 232 b, 232 c, each having a hollowsocket 264 with one or more oval-shaped spring members 266 in eachsocket. A fourth sidewall 232 d, which represents the anterior side ofinterbody 210 after insertion, forms a large tab 236. Upper plate 220has three sidewalls 222 a, 222 b, 222 c with plug extensions 262. Afourth sidewall 222 d has a recess 226 that receives tab 236 of lowerplate 230. Plug extensions 262 of sidewalls 222 a, 222 b, 222 c aretelescopically received in sidewalls 232 a, 232 b, 232 c, respectively.In this arrangement, upper and lower plates 220, 230 are permitted tocollapse in an axial direction relative to one another. Spring members266 provide a limited amount of resistance to axial compression so thatosteogenic material 280 in chamber 250 may be shielded from some of theaxial load during collapse.

Interbody 210 is configured to be compressed to a thin profile as shownin FIG. 8B to permit the interbody to be inserted into the disc space.After interbody 210 is inserted into the disc space, the interbody isconfigured to expand or self-distract. Interbody 210 can be compressedby applying axial pressure on upper plate 220 to advance the upper plateinto lower plate 230 and compress elastic members 266. To facilitatecompression, elastic members 266 may be formed of shape memory materialthat is inserted into the disc space at a reduced temperature, andsubsequently heated to expand the implant. Elastic members 266 may beexpanded in response to body temperature or external heat applied to theelastic members. Interbody 210 self-distracts as elastic members 266expand. After elastic members 266 are fully expanded, they remainflexible to adjust to changes in load on the interbody. As axial loadincreases, elastic members 266 flex under load, allowing upper plate 220to collapse into lower plate. Elastic members 266 absorb some of theload, while allowing some of the load to be applied to osteogenicmaterial in chamber 250. The height of chamber 250 decreases by anamount corresponding to the amount of collapse. The interior walls oflower plate 230 remain stationary, so that the volume of chamber 250decreases as upper plate 220 collapses into lower plate 230. Thestationary walls of lower plate 230 confine the osteogenic material andprevent lateral displacement of the material. In this arrangement,collapse of the upper plate 220 into lower plate 230 increaseshydrostatic pressure in the graft chamber. The geometry and material ofelastic members 266 may be selected to permit a desired range ofcollapse and increase in hydrostatic pressure.

Referring now FIG. 9, another exemplary interbody 310 is shown inaccordance with the present invention. Interbody 310 includes an annularbody 320 that forms an inner wall 340 surrounding a central chamber 350.Chamber 350 contains an osteogenic material 380. Interbody 310 furtherincludes a contraction mechanism provided by a shape memory polymer 322in annular body 320. Shape memory polymer 322 is designed to contractover time to create hydrostatic pressure in the chamber 350. Annularbody 320 may be formed to contract strictly in response to time, thetransfer of heat, or both.

Annular body 320 may be designed to contract in the axial direction,radial direction, or a combination of both directions to apply andmaintain hydrostatic pressure to osteogenic material 380 in chamber 350.Referring now FIGS. 10A and 10B, interbody 310 is configured to contractin both the axial and radial directions over time. FIG. 10A showsinterbody 310 in an intraoperative state, and FIG. 10B shows the sameinterbody in a post-settling state. Interbody 310 contracts both axiallyand radially during settling, decreasing the volume of chamber 350.Osteogenic material 380 is confined between the adjoining vertebralbodies and within annular body 320. As a result, hydrostatic pressure inchamber 350 increases in response to contraction of interbody 310.

In some circumstances, it may be desirable to provide an interbody thatcontracts only in the radial direction to apply hydrostatic pressure inthe radial direction. Referring now FIGS. 11A and 11B, another exemplaryinterbody 410 is shown in accordance with the present invention thatcontracts only in the radial direction. Interbody 410 includes anannular body 420 containing a shape memory polymer 422.

Although the embodiments described above are discussed with specificexamples of contraction mechanisms, including elastic members and shapememory polymers, a number of materials may be used to allow theinterbody to change from a desired pre-implantation configuration to apost-implantation configuration. As noted above, the interbody mayinclude a shape memory material, such as a shape memory metal, ceramicor polymer, that is inserted into a disc space or other bone recess in apre-implantation shape, and then activated into a post-implantationshape. A number of shape memory materials, many of which may be used inaccordance with the present invention, are described in InternationalPub. No. WO 2006/108114, the contents of which is incorporated byreference in its entirety. Interbodies in accordance with the presentinvention may also include elastomers, mechanical spring members or anyother materials that can deform to a desired post-implantation shape.

While preferred embodiments of the invention have been shown anddescribed herein, it will be understood that such embodiments areprovided by way of example only. Numerous variations, changes andsubstitutions will occur to those skilled in the art without departingfrom the spirit of the invention. Accordingly, it is intended that theappended claims cover all such variations as fall within the spirit andscope of the invention.

1. An intervertebral implant having an upper surface configured forengagement with a first vertebral body, a lower surface configured forengagement with a second vertebral body, a wall extending between theupper surface and the lower surface, and a chamber enclosed within thewall, the chamber having an upper end opening through the upper surface,and a lower end opening through the lower surface, the wall comprising acollapsible section between the upper surface and the lower surface, thecollapsible section being collapsible from a first position associatedwith a first volume of the chamber to a second position associated witha second is volume of the chamber, the second volume being less than thefirst volume.
 2. The intervertebral implant of claim 1, wherein thecollapsible section of the wall comprises a first wall section and asecond wall section telescopically received in the first wall section.3. The intervertebral implant of claim 2, wherein the first and secondwall sections comprise non-circular geometries that limit rotation ofthe first wall section relative to the second wall section.
 4. Theintervertebral implant of claim 1, wherein the collapsible section ofthe wall comprises a spring member.
 5. The intervertebral implant ofclaim 4, wherein the spring member comprises a shape memory material. 6.The intervertebral implant of claim 5, wherein the spring membercomprises a shape memory polymer deformable between a pre-implantationshape and a post-implantation shape, the chamber having a first volumewhen the spring member assumes the pre-implantation shape, and thechamber having the second volume when the spring member assumes thepost-implantation shape.
 7. The intervertebral implant of claim 1,wherein the collapsible section of the wall is collapsible solely in theradial direction.
 8. An intervertebral implant comprising an upper plateconfigured for engagement with a first vertebral body, a lower plateconfigured for engagement with a second vertebral body, and a chamberbetween the upper plate and the lower plate, the chamber surrounded by afixed inner wall in the upper plate and a fixed inner wall in the lowerplate, the upper plate axially movable toward the lower plate to reducethe volume of the chamber and increase the hydrostatic pressure on aosteogenic material in the chamber.
 9. The intervertebral implant ofclaim 8, wherein the upper plate is telescopically received in the lowerplate.
 10. The intervertebral implant of claim 9, wherein the upper andlower plates comprise non-circular geometries that limit rotation of theupper plate relative to the lower plate.
 11. The intervertebral implantof claim 8, wherein one of the fixed inner walls comprises a contractionmember embedded in said wall.
 12. The intervertebral implant of claim11, wherein the contraction member comprises a spring.
 13. Theintervertebral implant of claim 11, wherein the contraction membercomprises a shape memory polymer deformable between a pre-implantationshape and a post-implantation shape, the chamber having a first volumewhen the contraction member assumes the pre-implantation shape, and thechamber having the second volume when the contraction member assumes thepost-implantation shape, the second volume being less than the firstvolume.
 14. The intervertebral implant of claim 8, wherein thecontraction member comprises a shape memory material that is collapsiblein the radial direction in response to temperature increase in the wall.15. An intervertebral implant comprising an upper plate configured forengagement with a first vertebral body, a lower plate configured forengagement with a second vertebral body, a chamber between the upperplate and the lower plate, and an osteogenic material under ahydrostatic pressure in the chamber, the upper plate axially movabletoward the lower plate to reduce the volume of the chamber and increasethe hydrostatic pressure on the osteogenic material in the chamber. 16.The intervertebral implant of claim 15, wherein the upper plate istelescopically received in the lower plate.
 17. The intervertebralimplant of claim 16, wherein the upper and lower plates comprisenon-circular geometries that limit rotation of the upper plate relativeto the lower plate.
 18. The intervertebral implant of claim 15, whereinat least one of the upper and lower plates comprises a spring member.19. The intervertebral implant of claim 18, wherein the spring membercomprises a shape memory material.
 20. The intervertebral implant ofclaim 18, wherein the spring member comprises a shape memory polymerdeformable between a pre-implantation shape and a post-implantationshape, the chamber having a first volume when the spring member assumesthe pre-implantation shape, and the chamber having the second volumewhen the spring member assumes the post-implantation shape, the secondvolume being less than the first volume.
 21. The intervertebral implantof claim 15, wherein the wall is collapsible in the radial direction.22. An intervertebral implant comprising a body formed of a shape memorymaterial, the body being deformable in response to temperature from apre-insertion configuration to a post-insertion configuration, the bodyforming a chamber and comprising an osteogenic material in the chamber,the chamber having a first volume and exerting a first hydrostaticpressure on the osteogenic material in the pre-insertion configuration,the chamber having a second volume and exerting a second hydrostaticpressure on the osteogenic material in the post-insertion configuration,the second volume being less than the first volume and the secondhydrostatic pressure being greater than the first hydrostatic pressure.23. The intervertebral implant of claim 22, wherein the body contractsaxially and radially from the pre-insertion configuration to thepost-insertion configuration.
 24. The intervertebral implant of claim22, wherein the body contracts only radially from the pre-insertionconfiguration to the post-insertion configuration.